RF coils are an essential part of magnetic resonance systems both in spectroscopy and in imaging. Many different types of RF coils are used. For example, body coils, large enough to receive a patient therein, which are usually integral to the magnet system are used. Special surface coils designed to acquire data from specific parts of the body are also used. For example, if a mammalary scan is being carried out special coils for that type of examination will ordinarily be used.
As anyone who has worked with microwave or high RF frequencies knows, there are always balancing problems inherent at the high frequencies; especially, when coupling balanced and unbalanced circuits together. The RF coils in MR systems are generally connected to the MR system, and more particularly to the the RF generator and/or the RF receiver of the MR system using coaxial cable. Coaxial cable is necessary to protect the system from "picking-up" extraneous RF signals which are present in the environment. As is well known coaxial cable, features a braid separated from a current carrying central conductor by a plastic material. The braid acts as a shield that minimizes the pick-up of foreign frequencies by the cable.
Although coaxial cable is used, nonetheless, there are still coupling problems at very high frequencies. Amount other things the braiding itself tends to carry foreign induced currents. The induced current is often referred to as "skin current" since it flows on the outside of the braiding. The skin current tends to flow into the RF coil and upset the measurements being taken.
It is well known that the signals measured in MR systems are extremely small and accordingly, anything that disturbs the "Q" of the coil or adversly effects the signal to noise ratio (SNR) of the system has disastrous effects. The adverse effects of the foreign currents are especially pronounced in the special surface coils which are connected to the system, through relatively long cable sections and therefore are prone to radiate and receive foreign RF and/or noise signals.
Balance/unbalance (Balun) circuitry is used as one means for reducing the noise and/or foreign currents generated due to the induced currents in the coaxial cable. In the prior art these balun circuits have included tuned elements for filtering out or preventing the induced currents from disturbing the delicate data measurements.
The tuned elements, of course, introduce their own problems. For example, the use of tuned elements increases the cost of the system. In addition the use of extra elements has an inherent adverse effect on the reliability of the system. The stability of the added elements always has to be considered.
Accordingly, more efficient balun systems for reducing the interference caused by induced currents and noise picked up in the connecting cables of the RF coils or probes is something which those using RF coils in MR systems are constantly seeking.